set in a universre where people are more cat like - man cat and a princess cat have to ultimately stay in a spaceship at the speed of light to gain back some mass that was somehow removed so the universe would keep expanding - anyone recall?

It isn't exactly what you described, but could you be thinking of Larry Niven's Ringworld?  They involve a feline-like race called Kzinti.  Wikipedia has a full plot synopsis: http://en.wikipedia.org/wiki/Ringworld